8 
DIADECTID&. 
IT have obtained three skulls of the Bmpedyveles molaris, a species of this 
family, which display the occiput, and two of them the basis of the cran- 
ial and facial regions. From them I derive the following characters.* 
The relations of the quadrate and zygomatic arches are as in the Thero- 
morpha generally. The pterygoids extend to the quadrates, and the vomer 
bears teeth. The brain-case extends to between the orbits, and its lateral 
walls are uninterrupted by fissures from this point to near the origin of the 
os quadratum. There is an enormous frontoparietal foramen. The mode of 
_ connection with the atlas is peculiar. There is a plane facet on each side of 
the foramen magnum, which then expands largely below them. The bone 
which bounds it inferiorly, presents on its posterior edge a median concavity. 
Oneach side of this, is a transverse cotylus, much like those of an atlas which 
are applied to the occipital condyles of the Mammalia. They occupy pre- 
cisely the position of the Mammalian condyies. The median point of their 
upper border, which forms the floor of the foramen magnum, is produced 
in the position occupied by the median occipital condyle of a reptile. 
From its position between the cotyli, the section of this process is triangular. 
’ The element in which the cotyli are excavated has the form of the mam- 
malian basioccipital, and of the reptilian sphenoid. It is not the batrachian 
parasphenoid. -Its extreme external border on each side where it joins a 
crest descending from the exoccipital, is excavated by a circular fossa 
which looks outwards. 
The character of this articulation is so distinct from anything yet known 
among vertebrated animals, that I felt justified in proposing (1. ¢., p. 804) 
a new division of the Theromorpha to include the Diadectidw, to be called 
the Cotylosauria. The superior facets described, indicate the presence of 
atlantal zygapophyses as in the Ganocephala, 
There are three genera of Diadectide, one of which is now introduced 
for the first time. They are distinguished as follows : 
I. Molar teeth in one series ; 
A distinct canine........... cesses ee eee a . Diadectes. 
No canine............ saatnicss SLES aalncwnanldneenacued Empedocles. 
JI. Molar teeth in two series; | 
SACOM C aicivelacg os'e'd 4 steetiey VS atenwaligtuitrs.n ellaswinies Helodectes. 
I am acquainted with six species of this family, two of each of the genera. 
DIADECTES Cope. 
Proceeds. Amer. Philos. Society, 1878, p. 505. American Naturalist, 
April 22, 1878. ; 
The typical species of the genus has compressed teeth, with one end of 
the crown much more elevated than the other. In the lower jaw the inner 
extremity is the elevated one, and vice versa. There is a large tooth in the 
position of a canine in the inferior series, but it is not certain whether or 
not it isan incisor. A new species is now described which is intermediate 
*These were first described in the American Naturalist, 1889, p. 301. 
